Continuing the discussion from Understanding groups and category permissions:
In some forums it could encourage lurkers to sign up (and reduce the barrier to subsequent participation) if the first post were visible to anonymous uses, but replies only visible to registered users. (There might be other variations based on specific groups.)
I also wonder whether it would help reduce AI scraping, but maybe that is futile…
I’m aware of Gated Topics in Category but have in mind something that doesn’t send the full page to the browser at all. And shows the full first post rather than just a percentage of the screen.